A moving coil geophone (Figure 3.17) operates according to the principle of a microphone
or a loudspeaker: the coil consisting of copper wire wound on a thin non-conducting
cylinder ("former") moves in the ring-shaped gap of a magnet. Figure 3.17 is the cross
section of a cylindrical structure. The annular magnet and polar pieces N and S in soft
iron create a radial field in the gap. The only movement allowed for the coil, suspended
from springs not shown in the picture, is a translation along the direction of the axis and
in the gap. As the coil moves, its windings cut magnetic lines of force and an electromotive
force is generated. The output voltage is proportional to the rate at which the coil cuts
the lines of magnetic force, that is to say, proportional to the velocity at which it moves.
Therefore this type of detector is known as "velocity geophone".